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16-1 John Maynard Keynes and the Consumption Function CHAPTER SIXTEEN Consumption16 Consumption is the sole end and purpose of all production. — Adam Smith How do households decide how much of their income to consume today and how much to save for the future? This is a microeconomic question because it addresses the behavior of individual decisionmakers.Yet its answer has macroeco- nomic consequences. As we have seen in previous chapters, households’ con- sumption decisions affect the way the economy as a whole behaves both in the long run and in the short run. The consumption decision is crucial for long-run analysis because of its role in economic growth.The Solow growth model of Chapters 7 and 8 shows that the saving rate is a key determinant of the steady-state capital stock and thus of the level of economic well-being.The saving rate measures how much of its in- come the present generation is putting aside for its own future and for future generations. The consumption decision is crucial for short-run analysis because of its role in determining aggregate demand. Consumption is two-thirds of GDP,so fluctu- ations in consumption are a key element of booms and recessions.The IS–LM model of Chapters 10 and 11 shows that changes in consumers’ spending plans can be a source of shocks to the economy, and that the marginal propensity to consume is a determinant of the fiscal-policy multipliers. In previous chapters we explained consumption with a function that relates consumption to disposable income: C = C(Y − T ).This approximation allowed us to develop simple models for long-run and short-run analysis, but it is too simple to provide a complete explanation of consumer behavior. In this chapter we examine the consumption function in greater detail and develop a more thorough explanation of what determines aggregate consumption. Since macroeconomics began as a field of study,many economists have writ- ten about the theory of consumer behavior and suggested alternative ways of interpreting the data on consumption and income. This chapter presents the views of six prominent economists to show the diverse approaches to explain- ing consumption. 432 | User SONPR:Job EFF01432:6264_ch16:Pg 432:28150#/eps at 100% *28150* Wed, Feb 20, 2002 3:12 PM CHAPTER 16 Consumption | 433 16-1 John Maynard Keynes and the Consumption Function We begin our study of consumption with John Maynard Keynes’s General The- ory, which was published in 1936. Keynes made the consumption function cen- tral to his theory of economic fluctuations, and it has played a key role in macroeconomic analysis ever since. Let’s consider what Keynes thought about the consumption function, and then see what puzzles arose when his ideas were confronted with the data. Keynes’s Conjectures Today, economists who study consumption rely on sophisticated techniques of data analysis.With the help of computers, they analyze aggregate data on the be- havior of the overall economy from the national income accounts and detailed data on the behavior of individual households from surveys. Because Keynes wrote in the 1930s, however, he had neither the advantage of these data nor the computers necessary to analyze such large data sets. Instead of relying on statisti- cal analysis, Keynes made conjectures about the consumption function based on introspection and casual observation. First and most important, Keynes conjectured that the marginal propensity to consume—the amount consumed out of an additional dollar of income—is between zero and one. He wrote that the “fundamental psychological law, upon which we are entitled to depend with great confidence,...is that men are dis- posed, as a rule and on the average, to increase their consumption as their income increases, but not by as much as the increase in their income.’’ That is, when a person earns an extra dollar, he typically spends some of it and saves some of it. As we saw in Chapter 10 when we developed the Keynesian cross, the marginal propensity to consume was crucial to Keynes’s policy recommendations for how to reduce widespread unemployment.The power of fiscal policy to influence the economy—as expressed by the fiscal-policy multipliers—arises from the feed- back between income and consumption. Second, Keynes posited that the ratio of consumption to income, called the average propensity to consume, falls as income rises. He believed that saving was a luxury,so he expected the rich to save a higher proportion of their income than the poor.Although not essential for Keynes’s own analysis, the postulate that the average propensity to consume falls as income rises became a central part of early Keynesian economics. Third, Keynes thought that income is the primary determinant of consumption and that the interest rate does not have an important role.This conjecture stood in stark contrast to the beliefs of the classical economists who preceded him. The classical economists held that a higher interest rate encourages saving and discour- ages consumption. Keynes admitted that the interest rate could influence con- sumption as a matter of theory.Yet he wrote that “the main conclusion suggested User SONPR:Job EFF01432:6264_ch16:Pg 433:28151#/eps at 100% *28151* Wed, Feb 20, 2002 3:12 PM 434 | PART VI More on the Microeconomics Behind Macroeconomics figure 16-1 Consumption, C The Keynesian Consumption Function This figure graphs a = + consumption function with C C cY the three properties that Keynes conjectured. First, the marginal propensity to con- sume c is between zero and MPC 1 one. Second, the average C APC propensity to consume falls as income rises. Third, con- 1 APC sumption is determined by 1 current income. Income, Y Note: The marginal propensity to consume, MPC, is the slope of the consumption function. The average propensity to consume, APC = C/Y, equals the slope of a line drawn from the origin to a point on the consumption function. by experience, I think, is that the short-period influence of the rate of interest on individual spending out of a given income is secondary and relatively unimportant.’’ On the basis of these three conjectures, the Keynesian consumption function is often written as − − C = C + cY, C > 0, 0 < c < 1, − where C is consumption, Y is disposable income, C is a constant, and c is the marginal propensity to consume. This consumption function, shown in Figure 16-1, is graphed as a straight line. Notice that this consumption function exhibits the three properties that Keynes posited. It satisfies Keynes’s first property because the marginal propen- sity to consume c is between zero and one, so that higher income leads to higher consumption and also to higher saving. This consumption function satisfies Keynes’s second property because the average propensity to consume APC is − APC = C/Y = C/Y + c. − As Y rises, C/Y falls, and so the average propensity to consume C/Y falls.And fi- nally, this consumption function satisfies Keynes’s third property because the in- terest rate is not included in this equation as a determinant of consumption. The Early Empirical Successes Soon after Keynes proposed the consumption function, economists began col- lecting and examining data to test his conjectures.The earliest studies indicated that the Keynesian consumption function is a good approximation of how con- sumers behave. User SONPR:Job EFF01432:6264_ch16:Pg 434:28152#/eps at 100% *28152* Wed, Feb 20, 2002 3:12 PM CHAPTER 16 Consumption | 435 In some of these studies, researchers surveyed households and collected data on consumption and income.They found that households with higher income consumed more, which confirms that the marginal propensity to consume is greater than zero. They also found that households with higher income saved more, which confirms that the marginal propensity to consume is less than one. In addition, these researchers found that higher-income households saved a larger fraction of their income, which confirms that the average propensity to consume falls as income rises. Thus, these data verified Keynes’s conjectures about the marginal and average propensities to consume. In other studies, researchers examined aggregate data on consumption and in- come for the period between the two world wars.These data also supported the Keynesian consumption function. In years when income was unusually low,such as during the depths of the Great Depression, both consumption and saving were low,indicating that the marginal propensity to consume is between zero and one. In addition, during those years of low income, the ratio of consumption to in- come was high, confirming Keynes’s second conjecture. Finally,because the cor- relation between income and consumption was so strong, no other variable appeared to be important for explaining consumption.Thus, the data also con- firmed Keynes’s third conjecture that income is the primary determinant of how much people choose to consume. Secular Stagnation, Simon Kuznets, and the Consumption Puzzle Although the Keynesian consumption function met with early successes, two anomalies soon arose. Both concern Keynes’s conjecture that the average propen- sity to consume falls as income rises. The first anomaly became apparent after some economists made a dire—and, it turned out, erroneous—prediction during World War II. On the basis of the Keynesian consumption function, these economists reasoned that as incomes in the economy grew over time, households would consume a smaller and smaller fraction of their incomes.They feared that there might not be enough profitable investment projects to absorb all this saving. If so, the low consumption would lead to an inadequate demand for goods and services, resulting in a depression once the wartime demand from the government ceased. In other words, on the basis of the Keynesian consumption function, these economists predicted that the economy would experience what they called secular stagnation—a long de- pression of indefinite duration—unless fiscal policy was used to expand aggre- gate demand.
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